In so-called high-voltage MOS transistors, there is a tendency that heavy concentration of electric field takes place in the vicinity of drain edge of the channel region, and thus, the breakdown voltage at the drain edge is an important issue. Such a high-voltage MOS transistor may be used in various applications such as in-vehicle applications, power applications, and the like.
It is preferable to integrate such a high-voltage MOS transistor in the form of an integrated circuit together with logic circuits, and the like. On the other hand, when such a high-voltage MOS transistor is integrated with logic circuits, the gate insulation film is formed to have a small thickness similarly to the transistors of logic circuits. Thus, in such a high-voltage MOS transistor, there is used a construction in which the drain region is formed with large separation from the gate electrode such that there is formed a drift region between the gate electrode and the drain region for improving the breakdown voltage in the vicinity of the drain edge. For example, there is a so-called LDMOS (Laterally Diffused MOS) structure. Reference should be made to the Patent References 1 and 2 and further the Non-Patent Reference 1.
With a high-voltage MOS transistor of the LDMOS structure, there is a demand for reducing the ON-resistance and at the same time to improve the breakdown voltage for enabling operation under further higher voltage.
With the construction of the Patent Reference 1 or 2, the gate insulation film is formed to have an increased film thickness at the drain edge of the gate electrode for improvement of the transistor breakdown voltage. With such a construction, there is a need for the carriers, flowing from the source region to the drain region, to flow under another insulation film formed underneath the gate insulation film along a flow path circumventing the another insulation film, while such a circumventing flow path of the carriers causes the problem of increased ON-resistance and also the problem of concentration of electric field associated with the bend of the carrier flow path underneath the another insulation film. When such concentration of electric field takes place, the breakdown voltage tends to be degraded.